What is the difference between Nullable.HasValue or Nullable != null?

I always used Nullable<>.HasValue because I liked the semantics. However, recently I was working on someone else’s existing codebase where they used Nullable<> != null exclusively instead.

Is there a reason to use one over the other, or is it purely preference?

1. int? a;
   if (a.HasValue)
       // ...
   

vs.

2. int? b;
   if (b != null)
       // ...